
It depends entirely on the dog. Some of mine would not like being singletons, a couple of others would be better that way (but aren't rehomeable).
Some may love other dogs but can be too much for them, too - I walk a little JRT X chi who LOVES other dogs but is absolutely best off living as an only one, because he is far too pushy and annoying. I had him here for a few weeks and it is just incessant. A friend met him while he was here and she said the same - clearly loves them but absolutely, best kept alone.
By contrast though, I also fostered a dog a few years ago who absolutely should not be an only dog, and it was a stipulation of his adoption that he must live with at least one other.
And it may be more specific than that - my boy now likes being with others, and he'll play with them, but what he really wants is another male (preferably big and fluffy) to adore. He was head over heels in love with that foster dog!
Perhaps fostering would be a good idea for you? Quite a few rescues offer a 'foster with a view to adopt' system now, so you can take a dog on without fully committing to keeping them, and see how it goes with your JRT before anything becomes permanent.